Spatial tuning to virtual sounds in the inferior colliculus of the guinea pig
S.J. Sterbing, K. Hartung & K.-P. Hoffmann
J.Neurophysiol., 90: 2648-2659, 2003
- How do neurons in the inferior colliculus (IC) encode the spatial location of sound? We have addressed this
question using a virtual auditory environment. For this purpose, the individual head-related transfer functions
(HRTFs) of 18 guinea pigs were measured under free-field conditions for 122 locations covering the upper hemisphere.
From 257 neurons, 94% responded to the short (50-ms) white noise stimulus at 70 dB sound pressure level (SPL).
Out of these neurons, 80% were spatially tuned with a receptive field that is smaller than a hemifield (at 70 dB).
The remainder responded omnidirectionally or showed fractured receptive fields. The majority of the neurons preferred
directions in the contralateral hemisphere. However, preference for front or rear positions and high elevations
occurred frequently. For stimulation at 70 dB SPL, the average diameter of the receptive fields, based on half-maximal
response, was less than a quarter of the upper hemisphere. Neurons that preferred frontal directions responded
weakly or showed no response to posterior directions and vice versa. Hence, front/back discrimination is present
at the single-neuron level in the IC. When nonindividual HRTFs were used to create the stimuli, the spatial receptive
fields of most neurons became larger, split into several parts, changed position, or the response became omnidirectional.
Variation of absolute sound intensity had little effect on the preferred directions of the neurons over a range
of 20 to 40 dB above threshold. With increasing intensity, most receptive fields remained constant or expanded.
Furthermore, we tested the influence of binaural decorrelation and stimulus bandwidth on spatial tuning. The vast
majority of neurons with a low characteristic frequency (<2.5 kHz) lost spatial tuning under stimulation with
binaurally uncorrelated noise, whereas high-frequency units were mostly unaffected. Most neurons that showed spatial
tuning under broadband stimulation (white noise and 1 octave wide noise) turned omnidirectional when stimulated
with 1/3 octave wide noise.
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